Nonlinear, Resonance-Stabilized Pseudohalides: From Alkali Methanides to Ionic Liquids of Methanides


  • Dedicated to Professor Dr. Karl Otto Christe on the occasion of his 70th birthday


New 1-ethyl-3-methylimidazolium (EMI) salts [EMI][X] (X = [HC(NO2)(CN)], [C(NO2)(NO)(CN)], [HC(NO)2] and [HC(NO2)2]) and 1-n-butyl-3-methylimidazolium (BMI) salts [BMI][Y] (Y = [C(CN)3] and [C(NO2)(NO)(CN)]) were prepared and characterized. Different synthetic routes to these new resonance-stabilized methanide-based ionic liquids starting either from the explosive silver salts (except from Ag[C(CN)3]) or the easily accessible potassium salts have been described. The melting points of all new salts are lower than 100 °C, in fact, most of them are ionic liquids at room temperature. These strongly colored ionic liquids (besides [BMI][C(CN)3] which is colorless) are neither heat nor shock sensitive, are thermally stable up to over 52 °C ([EMI][C(NO2)(NO)(CN)]) and 270 °C ([BMI][C(CN)3]) and can be prepared in large quantities. The structure and bonding of resonance-stabilized methanides ([CR1R2R3] with, R1,2,3 = H, NO2, NO, and CN) is discussed on the basis of experimental and theoretical data. X-ray data of Cs+[HC(NO2)(CN)], K+[HC(NO2)2], [EMI]+[C(NO2)(NO)(CN)], and [Me4N]+[C(NO2)(NO)(CN)] reveal almost planar anions with strong cation···anion interactions in the alkali methanides resulting in three-dimensional network structures in the solid state. Only weak interionic interactions are found for the ammonium salts. As shown by different theoretical approaches (charge transfer, resonance energies and NLMO delocalization) resonance effects occur in all three classes of methanides (NO2-, NO-, and CN-substituted), however, the magnitude of such effect strongly differs depending on the degree of substitution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)